The present invention relates to a stuffing process and apparatus for reducing waiting time jitter, of the type in which:
a first digital signal (A) with a first bit rate is supplied to a synchronizer and the first digital signal is written into an elastic memory (ES) in the synchronizer with a first clock pulse (t.sub.A) having a pulse rate which corresponds to the first bit rate of the first digital signal (A); PA1 b. a second digital signal (B) is read out of the elastic memory (ES) at a second bit rate with a second clock pulse (t.sub.B) having a pulse rate which corresponds to the second bit rate of the second digital signal (B), whereby the elastic memory (ES) will have a mean loading; and PA1 c. in a frame counter (RZ), stuffing frames of the second digital signal (B) are generated, reading out of the elastic memory is controlled, and stuffing is performed as a function of a control parameter supplied to the frame counter. PA1 determining the mean loading of the elastic memory (ES) at a rate of once per stuffing frame of the second digital signal (B); PA1 producing an indication of a difference between the mean loading and a selected loading value for each stuffing frame of the second digital signal (B); PA1 deriving a sum of the difference indicated during an existing stuffing frame and all previous stuffing frames for which the steps of determining and producing were performed; PA1 deriving, from the difference indicated during an existing stuffing frame and the current sum, an estimated sum value for the next occurring stuffing frame on the assumption that stuffing will not be performed during the next occurring stuffing frame; PA1 comparing the estimated sum value with at least one threshold value and determining, as a function of this comparison, a control parameter value which can control stuffing in a manner such that the sum derived in the first-recited deriving step remains at least approximately constant; and PA1 supplying the value determined in the comparing step as the control value, to the frame counter (RZ). PA1 means for determining the mean loading of the elastic memory (ES) at a rate of once per stuffing frame of the second digital signal (B); PA1 means for producing an indication of a difference between the mean loading and a selected loading value for each stuffing frame of the second digital signal (B); PA1 means for deriving a sum of the difference indicated during an existing stuffing frame and all previous stuffing frames for which a mean loading was previously determined and an indication of a difference was previously produced; PA1 means for deriving, from the difference indicated during an existing stuffing frame and the current sum, an estimated sum value for the next occurring stuffing frame on the assumption that stuffing will not be performed during the next occurring stuffing frame; PA1 means for comparing a currently derived estimated sum value with at least one threshold value and for determining, as a function of this comparison, a control parameter value which can control stuffing in a manner such that the sum derived by the means for deriving remains at least approximately constant; and PA1 means for supplying the value determined by the means for comparing, as the control value, to the frame counter (RZ).
If a first asynchronous digital signal is intended to be inserted into a second digital signal, the basic problem of frequency adaption arises. This problem is solved by stuffing. A problem of the conventional stuffing process lies in that the first digital signal recovered by destuffing may contain a low-frequency jitter, waiting time jitter, which can no longer be removed. The size of this jitter is the result of the stuffing process employed.
The conventional process for positive stuffing and the size of waiting time jitter caused by this is known from D. L. Duttweiler, in "Waiting Time Jitter", The BELL system Technical Journal, Vol. 51, No. 1, 1972, pages 165 to 207. Waiting time jitter in the course of positive-null-negative-null stuffing is treated in an article by F. Kuhne, K. Lang, in "Positiv-Null-Negativ-Stopftechnik fur Multiplexubertragung plesiochroner Datensignale" [Positive-Null-Negative Stuffing Technology for Multiplex Transmission of Plesiochronous Data Signals], Frequenz [Frequency], Vol. 32, No. 10, 1978, pages 281 to 287.
It is known from this reference to supply a first digital signal with a first bit rate to a synchronizer and to write it there with a first pulse corresponding to the bit rate of the first digital signal into an elastic memory. A second digital signal is read out of the memory with a second pulse, the second clock rate corresponding to the bit rate of the second digital signal. The frame of the second digital signal is generated and the read activation of the elastic memory is controlled in counters. The mean loading of the elastic memory is determined once per frame of the second digital signal. A difference between the mean loading and the set value for the mean loading is formed for each stuffing frame of the second digital signal. It is known to compare the difference with a maximum deviation. If the difference is greater than the maximum deviation, stuffing is performed.
A process for reducing waiting time jitter in the course of positive-null-negative stuffing was described by D. CHOI in "Waiting Time Jitter Reduction", IEEE Transactions on Communications, Vol. 37, No. 11, 1989, pages 1231 to 1236. The author assumes that a nominal stuffing process from null, such as is the case in the positive-negative stuffing process, results in large waiting time jitter. Therefore, the proposed process initiates a stuffing ratio not equal to null and in this way achieves a reduction of waiting time jitter.
In the publication by W. D. Grover, T. E. Moore, J. A. McEachern in "Waiting Time Jitter Reduction by Synchronizer Stuff Threshold Modulation", GLOBECOM '87, pages 514 to 518, additional stuffing operations are provided for reducing waiting time jitter, which additional stuffing operations take place in such sequence that the effect is a frequency displacement of the jitter. It is only required for executing this process to provide a new control in the synchronizer for the time when stuffing is intended. While up to now stuffing was always performed when constant thresholds were exceeded in either direction, the thresholds are modulated in the present case.
This species-defining reference therefore proceeds from a method corresponding to that in the article by Kuhne and Lang. However, the method does not operate with fixed thresholds, instead, the thresholds are modulated.
The processes for reducing waiting time jitters described in these articles each have a free parameter, by means of which waiting time jitter is defined (for example the length of the periods in the case of threshold modulation). However, it is not permissible to choose this parameter in such a way that the smallest possible waiting time jitter results. Instead, the critical frequency of the PLL circuit of the desynchronizer must be taken into consideration. The larger this critical frequency, the more waiting time jitter must be allowed by means of the choice of the free parameter. If this is not taken into account, an additional jitter component is superimposed on the pulse generated by the desynchronizer, which is greater than the waiting time jitter.
There is a necessity of adapting the synchronizer and the desynchronizer structurally to each other on account of the free parameter. If the critical frequency of the PLL circuit is greater than expected, there will be increased jitter of the recovered pulse. But if in contrast to this the critical frequency of the PLL circuit is smaller than expected, this jitter is only negligibly reduced, because in this case waiting time jitter is dominant.